Dungeon Master's Guide
The Dungeon Master's Guide, abbreviated as DMG, is one of the three volumes that Wizards insists on republishing and revising every edition of Dungeons & Dragons. It basically tries to teach you how to be a good Dungeon Master (DM), despite the fact that good DMing comes from experience, not reading the Idiot's Guide to DMing.
The book also contains lists of magical items, rules of their creation and generation, additional rules that players can not be aware of, and lots of useful tables.
DMing can become quite a complicated task without a DMG, so it is advisable to have one in one form or another.